1. Field of the Invention
This invention relates to direct access storage devices having a rotating media. More particular, it relates to a direct access storage device (DASD) wherein a rotating storage medium interacts with a head so as to store and/or retrieve information.
2. Background Art
A head positioning servo system in a DASD performs three critical tasks. First, it moves the head from a given track to the proximity of a target track at optimum time using a velocity servo. Next, it positions the head on the target track with minimum settle-out time using a first type of position controller, such as, for example, a proportional derivative controller (PD). Finally, the servo system ensures minimum track-follow error using another type of position servo, such as, for example, a proportional integral derivative controller (PID).
The portable computer systems that employ fixed disk drives are prone to shock inputs arising from user mishandling. Above a certain shock level the fixed media in a DASD is forced to shift away from its ideal center of rotation causing an eccentricity referred to as disk-shift. In certain classes of storage devices, even in the absence of any external shock event, particularly in an optical drive with removable media, an eccentricity is present due to manufacturing tolerances. Any eccentricity in the rotating media produces a harmonic error component in the position error signal produced by the read/write head. Traditionally this is referred to as runout component, and it occurs at the spindle rotational frequency. One crucial problem is that of precise track-following in the presence of substantial eccentricity without compromising the settle-out time. To complicate matters, in a DASD with a plurality of disk-platters and heads, the disk-shift is expected to be of different magnitude for each platter. A low cost solution to this problem is desired.
Present practical track density or TPI (tracks per inch) for a magnetic disk drive is about 5 kTPI. This corresponds to a track pitch of 200 microinch. A disk shift of about 1000 microinch is easily produced when a shock level of the order of 500 g (g-gravity unit) is applied to a DASD, and it corresponds to 5 track-pitch of eccentricity. A conventional low bandwidth (400 Hz) servo system in a DASD has sufficient error rejection (about 20 dB) capability to compensate for eccentricity of the order of a fractional track pitch (e.g, 10% of track pitch) that may occur due to thermal expansion of the recording media, but will fail if substantial disk shift of the order of a track-pitch occurs due to an external shock event.
Since the eccentricity induced error component occurs at a known frequency with a measurable amplitude, feedforward servo methods can be used to reduce this error component. Feedforward methods assume that the actuator system has well defined dynamic characteristics, so that the expected error component can be minimized by producing an actuator motion in anticipation of this error. However, the actuator system is known to contain disturbance components that are not confidently predictable. For example, in the case of a rotary actuator system the pivot bearings do not behave as an ideal bearing system. The friction between sliding components becomes a source of uncertainty. Therefore, the feedforward methods are not robust in the sense that consistent track-follow performance is not assured. Further, the feedforward methods require relatively complex computations in order to produce the feedforward control signals. For example, a method proposed in U.S. Pat. No. 4,536,809 requires trigonometric manipulation of the position error signal in order to generate the control signal. In the so-called low end market, for example, in those disk drives used in personal computers, such feedforward schemes are difficult to implement in microcode because the microprocessors which implement control of the DASD tend to be less powerful than is required to perform complex arithmetic calculations.